The present invention relates to a power supply unit for the bipolar supply of power to a plasma technology system or a surface technology system as set forth in the preamble of claim 1. Such units, as are disclosed for example in EP 534 068 B, generally comprise a d.c. main power supply whose outputs are connected to the inputs of a bridge circuit of electronic power switches. The power switches are connected to control signal conditioning devices that control the power switches in the desired manner in order to obtain a desired pulse pattern for the plasma system. The unit has separate control signal conditioning devices to individually regulate the control times for the positive and negative output signals, allowing any impulse shape to be selected.
The object to the invention is to improve upon a prior-art power supply unit in such a way as to increase the degree of freedom in selecting a desired pulse shape. In the invention, this object is accomplished with a power supply unit of the type set forth in the preamble through the characteristic elements of claim 1. A further object of the invention is to create an apparatus that permits any desired pulse shapes to be supplied at frequencies into megahertz range. This object is accomplished by an apparatus as recited in claim 7.
The dependent claims set forth advantageous embodiments of the invention.
In the invention at least two d.c. power supplies must be used for each bridge circuit. The bridge is divided, insofar as a series circuit comprising two power semiconductors is interposed between the positive output of the first d.c. power supply and the negative output of the second d.c. power supply.
The same applies to the negative output of the first d.c. power supply and to the positive output of the second d.c. power supply, between which a series circuit comprising two power semiconductors is connected. The tap for the pulses sent to the plasma system always is located between the two power switches in the series circuits.
In this way, the amplitudes of the positive and negative pulses can be freely selectedxe2x80x94for example, corresponding to a desired signal pattern.
If, in addition, separate control signal conditioning devices are provided to individually control the various power switches, it is not only possible to select any desired amplitudes of the positive and negative pulses, it is also possible to select their switching time and signal pauses or dead times. Thus, the greatest possible degree of freedom for selecting a pulse pattern to be sent to a plasma system is provided.
Since these systems have to be able to switch a very high current, the effective range of the power supply units is limited to a frequency range of about 100 to 200 kHz. By using a plurality of power supply units connected in parallel, preferably two to eight, and synchronously controlling the individual power supply units with correspondingly short timing offsets, it is possible to achieve any desired pulse pattern with a frequency extending into the megahertz range if, for example, eight units having a frequency of 125 kHz are used. The devices are preferably controlled by means of a control bus at the control input of the individual units. In this case, the control signals from a central control unit are sent to the single individually addressable units.
To accomplish this, the individual power supply units are preferably provided with an address or ID code that allows the systematic response of each individual current power supply unit to be controlled.
The outputs of the d.c. power supplies are preferably stabilized by capacitative means using capacitors that have the highest possible capacitance in order to be able to provide very high pulse currents. If the power supply units are being operated near the limit of their maximum capacitance, the dead time between the pulses can be limited without constraints.
A bridge can preferably be connected between the negative outputs as well as between the positive outputs of each of the two d.c. power supply units, so that it is possible to switch to conventional operation, however such conventional operation does not permit the positive and negative pulse amplitudes to be individually controlled.
The maximum permissible current dynamics for the switchable transistors and free-wheeling diodes is adjusted by means of two output-side inductances L1 and L2. In this way the pulse current is read and evaluated dynamically. In particular, with very low-impedance short circuits, it is necessary to quickly determine the amount of excess current and to shut off the transistors as soon as possible, in order to prevent damage to the semiconductor layer, to the substrate surfaces, or to the plasma coating system.
The power supply unit of the invention or the apparatus comprised of power supply units, can be used for all plasma-technology processes such as CVD, plasma PVD, magnetron sputtering, plasma nitriding, and plasma etching.